The rearrangement of immunoglobulin heavy chain genes during B lymphocyte development may be the molecular basis for constant region switching, allelic exclusion, and VH diversification. Using recombinant DNA technology, these alterations will be explored in the germline and in malignantly transformed B cell tumors of mouse and human. We have cloned cDNA copies of mouse alpha, gamma 2b, and mu constant regions into plasmid and have determined most of the nucleotide sequences of these clones. These CH plasmids have been successfully used as hybridization probes for the isolation of the genomic counterparts from a bacteriophage "shotgun" collection of DNA fragments from mouse liver and fully differentiated plasma cell tumors. In the proposed mouse studies, the entire mRNA structure of the cDNA clones will be completed by (a) conventional DNA sequencing of the CH regions of the present clones and (b) by using the CH plasmids as primers to generate VH region cDNA clones for subsequent sequencing. Efforts will be made to isolate a delta(IgD) chain cDNA clone for structural analysis. The CH and VH plasmids constructed will be used concurrently as hybridization probes for isolation of additional genomic clones from the above shotguns and from shotguns prepared from DNA fragments of mouse B cells transformed at intermediate stages of development. Set of overlapping and otherwise appropriate genomic clones from the mouse and human shotgun collections will be characterized by DNA sequencing, heteroduplexing, and other physical techniques to assess the extent of structural rearrangement as a function of B cell maturation.